Go to www [dot] pearsonvue [dot] com and sign up for a web account. When they email you your username and password log into Pearson Vue and Select your Country, Exam etc and it will take you through a whole process in which you can view you exams price, book a test center and even pay online with a credit card.
If you don't come right just phone Pearson Vue and they should be able to direct you through the whole process.
Hope this helps. Sadly I had to make loads of calls and queries before I worked it out. There is no clear help on Oracle's website regarding individual exam bookings.
You might want to check whizlabs.com too. They might have an offer for a discounted exam voucher alongwith a Whizlabs mock exam. Their mock exams are pretty decent.
A few days I had got a promotional email from Whizlabs announcing the sale of Oracle Java Exam Vouchers at discounted prices (with free Retake Option in a few countries. There is no free retake option in India). In their own words - "Since we have limited vouchers please ensure that you are benefited by purchasing these vouchers before the stock ends. The validity of these vouchers is till 28th Feb 2012."